The sulfated octapeptide ##EQU2## has been found to possess cholecystokinin activity. As such it stimulates gall bladder contraction and is useful as a diagnostic aid in x-ray examination of the gall bladder in the same manner as cholecystokinin. For such purposes the sulfated octapeptide may be dissolved in water for injection to form an injectable which is administered either intravenously or subcutaneously to mammalian species, e.g., dogs or cats.